Residents in Sana'a said they were jolted from sleep as the bombardment began early on Tuesday and continued throughout the morning, shaking buildings and sending plumes of smoke over the city's skyline.

Students are in the middle of their end of year exams and parents took to social media to voice their fears as their children headed to school amid the attacks.

"My daughter Asma is having now her 9th grade national exam under heavy bombing in Sana'a. God protect Yemen," wrote Mohammed Al-Asaadi.

The Saudi-led coalition also announced a three-day closure of Sana'a's international airport, one of the beleaguered country's only lifelines to the outside world and a key entry point for humanitarian aid.

Residents in Sana'a said they took the airport closure as an ominous sign that the coalition intended heavy airstrikes in the coming days.

A ceasefire was agreed in April and - despite numerous violations - it led to a dramatic decrease in violence and civilian casualties and cleared the way for peace talks to begin in Kuwait.

Those talks broke down on Saturday after weeks of stalemate.

The Houthi rebels are fighting against the internationally-recognised government of YemenCredit:
EPA/YAHYA ARHAB

Both sides in the conflict have been accused of killing civilians and violating international law. But the UN has said the vast majority of civilian deaths have been caused by airstrikes from the coalition, which is supported by the US.

A report released this week by the Saudi-led coalition largely cleared itself of wrongdoing during eight high-profile bombings and concluded that all "safety procedures implemented by coalition forces adhered to international humanitarian law".

The report looked at a number of cases when the UN or humanitarian organisations have accused the coalition of killing civilians or bombing hospitals and humanitarian structures.

The report argued that in six of the eight cases the bombings had been totally justified because of military targets hiding in civilian areas, including hospitals.

In one of the bloodiest incidents, coalition aircraft struck a crowded market in March, killing 106 civilians, according to the UN. The coalition report concluded that there were "huge numbers" of Houthi rebels in the market, making it a justified target.

The UN said there was "no evidence" of military personnel inside the market and called the coalition's distinction making between civilians and military targets "at best woefully inadequate".